FWIK, HX machines tend towards the upper end for espresso extraction. Popular and economical are the e61 HX machines. Patented in 1961 the group has been around for a while and it's become a common part. Designed for temperature stability and preinfusion it's capable of very good espresso extraction.

Higher up the scale the HX machines start tending to proprietary group designs. Elektra A3 is one I can think of. Technically the A3 is not an e61 group, I believe.

While an HX isn't necessarily an e61 group I can't think of any others than the A3 that's an HX non e61. Non e61's are either single boiler thermostat controlled on the entry level end or double boiler direct connect saturated groups on the top end. I'd say advantages of e61 vs. non e61 are relative to price/design.

Advantage I've read of e61 groups is potentially easier, more complete extractions due to preinfusing forgiveness factor. Disadvantage of e16 groups is possibly blurred tasting espresso due to preinfusion. Non e61, more direct connect to the boiler machines can be more finicky at the entry level end. At higher ends they are considered to edge out e61 HX machines for extraction quality. Lack of preinfusion has been said to produce espresso with more defined character.

Many of the commercial HX machines are not E61 - for example the Nuova Simonelli machines do not use E61 groups. Also, the some of the finest double boiler machines use an entirely different group concept (LaMarzocco and Synesso machines use saturated groups).

The E61 group is a good design and that's why after 45 years it's still very popular, but other group designs can be just as good or even better.

Yeah, I was thinking of the Synesso Cyncra and LaMarzocco GS3 saturated groups but they're double boilers with PID control, aren't they?

The HB thread talks about solenoid controlled e61 'style' or 'cloned' groups that by definition aren't true e61's. It think the idea is that with the clones, since the water flow is controlled by the closing off of one solenoid the same preinfusion that takes place with a true lever controlled e61 group doesn't happen. With solenoid controlled e61 'clone' groups a gicleur can be added to control the ramping brew pressure to offer a form of preinfusion but the opinion is that even with the gicleur the ramping is not consistent between designs or often slow enough to mimic true e61 groups.

According to the HB thread a true e61 group as defined by the patent needs infuse with manually controlled alternately seating valves. So it looks like the definition of what is a true e61 group and non-e61 group needs to include the solenoid controlled e61 looking groups in the non-e61 category depending on what you're definition/expectation is of an e61 group.

Many of the commercial HX machines are not E61 - for example the Nuova Simonelli machines do not use E61 groups. Also, the some of the finest double boiler machines use an entirely different group concept (LaMarzocco and Synesso machines use saturated groups).

The E61 group is a good design and that's why after 45 years it's still very popular, but other group designs can be just as good or even better.

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